In many signal processing applications it is desired to measure the time integral of the individual pulses. For example this is the case in the measurement of gamma radiation with a scintillation detector. Each gamma quantum induces a scintillation in a scintillator crystal, that is, an emission of light. The emission of light is detected with a photomultiplier tube whose output current is proportional to the light intensity received. The intensity, and thus the output signal of the photomultiplier tube, decreases exponentially with time. The time constant of this exponential decay is determined by the properties of the scintillator crystal and may, acordingly, be measured in a calibration measurement. The time integral of the signal from the single scintillation is proportional to the amount of energy which the gamma quantum has released in the scintillator crystal.
In gamma cameras which are used for generating an image of the radiation distribution from a radioactive isotope which, for example (for diagnostic purposes) is introduced into the blood circulation of a patient, a larger number of photomultiplier tubes is arranged in front of a flat scintillator crystal which is common for all tubes. How much each tube "sees" of the light from a scintillation in the crystal depends on the position of the scintillation in relation to the tube. This implies that it is possible by means of an analysis of the output signals of the tubes to determine the scintillation position. From this it is possible, using a suitable collimator for the gamma radiation, to determine the position of emission of the gamma quantum which triggered the scintillation, and thus to construct an image of the isotope distribution.